The present invention relates to a new and improved construction of an ionization smoke detector.
Generally speaking, the ionization smoke detector of the present invention is of the type containing at least one ionization chamber operating at an extra low voltage, the ionization chamber containing a sensor having a measuring electrode and a counter electrode. The ambient air has practically free access to the ionization chamber and the latter is provided with at least one radioactive source for generating ions. An electrical circuit is provided for alarm triggering, and the smoke detector is connected by means of lines with a central signal station which delivers a detector-operating voltage to the lines.
As to the presently employed fire alarms known in this technology ionization smoke detectors are the ones most widely employed as early warning detectors. Certain of the primary advantages of such type smoke detectors are their universal applicability and their simple and robust mechanical construction. Since the fire alarm, in the event there is encountered a combustion process, must respond rapidly and positively, but on the other hand should not be triggered by any false alarms, high requirements are placed upon the operational reliability of such ionization smoke detectors. Examples of ionization fire alarms are those disclosed, for instance, in U.S. Pat. Nos. 3,714,641, 3,909,813 and 4,037,106.
The principle of operation of heretofore known ionization smoke detectors is predicated upon markedly reducing the ionic current flowing between both electrodes of the measuring chamber whenever smoke penetrates into such measuring chamber. At the present time there are predominantly employed two types of ionization smoke detectors:
1. The low-voltage smoke detectors which operate at an operating voltage of about 200 volts, for instance as exemplified by the apparatus disclosed in U.S. Pat. No. 3,233,100 serving to detect aerosols in gases; and
2. The extra low-voltage smoke detectors which operate with an operating voltage of less than 50 volts, for instance, the ionization fire alarm installation as described in U.S. Pat. No. 3,521,263.
The low-voltage smoke detectors use as the electrical amplifier element a cold-cathode tube. But however they have an appreciably greater signal-to-noise ratio than the extra low-voltage smoke detectors. In FIG. 1 there has been shown the circuitry of a typical low-voltage smoke detector wherein the measuring ionization chamber 10 is operated in series with a work resistor 20, preferably in the form of a saturated reference chamber. The connection point 15 or junction of both chambers 10 and 20 is connected with a control electrode 17 of a cold-cathode tube 25. The voltage drop across the measuring chamber 10, in the quiescent or rest condition, amounts to about 80 volts. When smoke penetrates into the measuring chamber 10 this voltage or potential increases by about 50 volts, and thus, reaches the ignition or firing voltage of the cold-cathode tube 25. This in turn causes a current flow to take place between the anode 27 and cathode 29, which can be suitably evaluated by means of a relay 30 for alarm triggering purposes.
Operational disturbances in ionization smoke detectors arise, on the one hand, because the detectors trip false alarms, or, on the other hand, the sensitivity of the detectors decreases during their operating time, which in an extreme case can result in a complete breakdown of the fire alarm. The low-voltage fire alarms of the previously described type are relatively insensitive to electrical disturbances which are captured by the line network acting as an antenna, since these disturbances or spurious signals must have an appreciable magnitude, i.e. must at least amount to 50 volts, in order to ignite the cold-cathode tube. Therefore, with such type detectors false alarms caused by electromagnetic disturbances are relatively seldom.
The chamber voltage of approximately 100 volts, needed for operating the low-voltage ionization smoke detectors, however, causes high electrical field intensities of several 100 V/cm to appear at the measuring electrode. The dust particles which are always present in air tend to electrostatically deposit at the electrodes. This in turn causes the electrodes to become coated with a dust layer which gradually becomes thicker. If such dust particles consist of electrically non-conductive materials, something which particularly frequently is the case in dry winter periods, then the ionic current within the measuring chamber is blocked and there can arise triggering of a false alarm. This makes it necessary that the fire alarm frequently be cleaned. But such work is associated with high costs.
With the availability of field-effect transistors it was possible to develop ionization smoke detectors which could be operated with an operating voltage of less than 50 volts. One such construction of ionization smoke detector of the extremely or extra low-voltage type has been described, for instance, in the aforementioned U.S. Pat. No. 3,521,263. In FIG. 2 of the accompanying drawings there has been illustrated circuitry of a typical extra low-voltage ionization smoke detector. The voltage appearing across the measuring chamber 35 simultaneously constitutes the gate voltage or potential for the field-effect transistor 40. This potential is chosen such that the transistor 40 is without current in its quiescent state. The controlled rectifier (SCR) therefore likewise, generally indicated by reference character 45, is blocked and the relay 50 is not energized. If the smoke or other combustion products enter into the measuring chamber 35 then the chamber voltage increases and upon exceeding a certain threshold value causes the firing of the SCR, so that the relay 50 triggers an alarm.
With such extra low-voltage ionization smoke detectors the change in potential at the measuring ionization chamber, needed for triggering an alarm, only amounts to a few volts. Since in the line network there can arise spurious pulses or signals of this order of magnitude, with this type of fire alarm there always exists the danger of false alarms. To compensate for this drawback there is needed an appreciable electronic circuit expenditure. On the other hand, as a compensating factor for this disadvantage is the positive benefit that with such type of fire alarms the danger of contamination is appreciably smaller owing to the considerably reduced field intensity.
Exceedingly high security requirements are placed upon fire alarm installations for obvious reasons. Up to the present it was not possible, in the case of ionization smoke detectors of the low-voltage type, to overcome the dust contamination danger, or in the case of the extremely or extra low-voltage fire alarms to elimate with simple means the susceptibility to electrical disturbances.